The invention relates to a colour display tube comprising an envelope consisting of a neck, a cone and a faceplate including a flat or slightly convex display window which changes into a skirt which is substantially parallel to the axis of the envelope via a sharply-curved transistion portion. A first set of points defining where the inner surface of the display window meets the sharply-curved inner surface of the transition portions forms a closed line 1. A second set of points defining where the outer surface of the display window meets the sharply-curved outer surface of the transition portion forms a closed line m. The display window has on its inner surface a substantially rectangular display screen which comprises a material luminescing in at least one colour. Means is provided in the neck to produce at least one electron beam.
The invention further relates to a display device comprising such a colour display tube.
Recently developed display tubes have flatter display windows, as described, for example, in Journal of Electronic Engineering, August, 1982, P. 24. This journal describes a display tube having a substantially rectangular display window having an outer contour which is slightly barrel-shaped. This does not be cause any problem with tubes which are placed in a cabinet and the outer circumference of which is concealed from the viewer by a bezel, because the inner edge of the bezel can adjoin the edge of the display screen. However for tubes having a display window which slightly projects beyond the cabinet (so-called push-through mounting), where a bezel can not be used, the substantially rectangular display screen on the inner wall of the much less rectangular display window dark areas above and below and on the left and on the right of the displayed picture. These areas vary in width and are annoying to the viewer.
Moreover, in said push-through mounted tubes a problem occurs in the vicinity of in the transistion portion of the faceplate, connecting the display window to the skirt. In this portion a lens is formed by the curvature of the glass, causing an annoying distortion of the edge of the display window for a viewer who views the picture at a large angle to the tube axis. So long as the boundary of the luminescent material of the display screen is situated at ample distance from the transition area, this problem does not occur. However, this has for its result that the display window in that case is not filled optimally by the display screen.
It is the object of the invention to provide a colour display tube the display screen of which fills the display window optimally and in which the annoying distortion of the edge of the displayed picture for a viewer who views the picture at the maximally desired viewing angle does not occur.
A display tube of the kind described in the opening paragraph is characterized according to the invention in that the boundary of the luminescent material of the display screen substantially coincides with the line 1, and from any point Q in the line m the shortest connection line to the line 1 encloses with a normal line h (normal at Q to the outer surface of the window) an angle .gamma., for which the relationship holds that: EQU n sin .gamma.=sin (.alpha.-.beta.)
wherein
n is the refractive index of the glass of the display tube, PA1 .alpha. is the maximum viewing angle, between 55.degree. and 65.degree., defined with respect to the tube axis, and PA1 .beta. is the angle between the normal line h and a line parallel to the tube axis.
The invention is based on the recognition of the fact that, if the above relationship is satisfied, the disturbing lens action which is formed by the transition area does not occur within the viewing angle .alpha..
In other words, the place where the outer surface of the window meets the sharply-curved transition surface is chosen with respect to the place where the inner surface of the window meets the inner, sharply-curved transition portion in such manner that the above relationship is satisfied. It should be noted that in flatter display windows having full display screens according to the invention edge distortion is more of a problem than in display tubes having more convex display windows.
A further embodiment of a display tube according to the invention is characterized in that the outer circumference of the display window is substantially parallel to the boundary of the luminescent material of the display screen. The advantage hereof is that a dark edge which is uniform in width is obtained around the display screen. The known barrel-shaped outer contour was assumed to be necessary in connection with the stringent requirements as regards implosion safety for the tube. Experiments and comparative calculations have demonstrated that a display window having a substantially rectangular outer contour does not lose its implosion safety under either static or dynamic loads as compared with the known tube which has a substantially flat display window and a barrel-shaped contour.
The invention is of importance for tubes having black matrix material between the elements of luminescent material of the display screen, where the matrix material extends to beyond the boundary of the luminescent material of the display screen. However, the invention is also of importance for tubes without such matrix material in which an aluminium film ("metal backing") is provided over the luminescent material and extends to beyond the boundary of the luminescent material. In fact, in the former case a wider darker edge is formed without using the invention and in the second case a shining edge (aluminium) is formed which is surrounded by a dark edge caused by the skirt.
By using the invention, only a narrow dark edge is obtained around the rectangular display screen which is everywhere equally wide and even emphasises the rectangularity of the display screen. Moreover, the narrow dark edge which has substantially the same width everywhere, in an operating tube leads to a picture presentation which is attractive to the viewer. Notably, the edge does not lead to a perceptive distortion of, for example, a number of straight colunns of digits displayed on the display screen. In a tube which is not in operation the narrow edge which has substantially the same width everywhere around the display screen leads to an aesthetic design.